a) Field of the Invention
The invention is directed to an arrangement for the detection of disk-shaped objects in a cassette, preferably for detecting the presence and the position of semiconductor wafers in a cassette with a plurality of shelves of identical size. It is applied particularly in the semiconductor industry for sensing the loading state of cassettes between individual technical steps in chip fabrication, preferably before, during or after access by a handling system.
b) Description of the Related Art
When processing disk-shaped objects, that is, substrates such as semiconductor wafers for the fabrication of integrated circuits, the substrates are generally arranged together by groups in cassettes, as they are called, so that they can be handled by automatic equipment. Because of the large number of technical steps which render semiconductor wafers of the type mentioned above expensive, damage resulting in unusable substrates due to errors in the handling of the substrates, for example, by double occupancy of a cassette shelf (omission of a processing step), empty shelves (carrying out the same processing step twice for the preceding or following shelf) and slanted positioning of substrates over two or more shelves (mechanical damage) should be excluded or minimized. Further, the handling system can work more efficiently and wasted routines can be prevented through exact knowledge of the actual positions of the semiconductor wafers.
A device with a purpose such as that mentioned above is known from U.S. Pat. No. 5,418,382. This device has illumination which is elongated in a slit-shaped manner and which is arranged along the stack of wafers in the vicinity of the edges of the wafers, a linear light sensor arranged in the same direction and optical transmission elements which, in the form of a number of light conducting rods, transmit the light reflected by the edges of the wafers onto the sensor lines which are set back relative to the illumination.
This simple construction is disadvantageous in that, on the one hand, a discontinuity present in the edge of a wafer for technical reasons, that is, the notch for detecting the orientation of the wafer, poses a severe problem for detection of the presence of a wafer because this notch "swallows" virtually all of the light of the slit-shaped illumination so that the sensor indicates the absence of a wafer and the omission of a processing step can accordingly lead to unusable wafers.
Further, since its edge is usually rounded in a standardized manner, a wafer exhibits a small component of specular reflected illumination light and, in the case of substrates having edges of optional or unknown quality (polished edges, rough edges or edges rounded in an indefinite manner), changing reflection behavior must even be taken into account. On the other hand, in the above-mentioned solution this results in the disadvantage that the optical transmission elements must advance very close to the edge of the wafer. However, it is desirable to maintain openness in this location to enable access of the handling system without the need for temporally and spatially separate optical detection.